Detailed explanation of the definition of PHP constants

This article is a detailed analysis and introduction to the definition of php constants. Friends who need it can refer to

Constants can be understood as variables with unchanged values. Once a constant value is defined, it cannot be changed anywhere else in the script. A constant consists of English letters, underscores, and numbers, but numbers cannot appear as the first letter.
Use define()function in php to define constants. The syntax format of this function is:

define(string constant_name, mixed value, case_sensitive = true)

This function has 3 parameters:
constant_name: required parameter, constant name, that is, identifier
value: required parameter, constant value
case_sensitive: optional Parameter, specifies whether to be case sensitive, set to true to indicate insensitivity

There are two ways to obtain a constant value:
1. Use The constant name directly obtains the value;
2. Use the constant() function.

The sample code is as follows:

<?php
  define ("MESSAGE", "PHP常量定义,常量名区别大小写");
  echo MESSAGE."<br/>";    //输出常量MESSAGE
  echo Message."<br/>";    //输出"Message"，表示没有该常量

  define("MESSAGE2", "PHP常量定义,常量名不区别大小写", true);
  echo MESSAGE2."<br/>";   //输出常量MESSAGE2
  echo Message2."<br/>";   //输出常量

  $constant_name = "message2";
  echo constant($constant_name)."<br/>";   //输出常量MESSAGE2
  echo defined("MESSAGE")."<br/>";         //如果定义返回true,echo输出显示1
 ?>

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